1. Field of the Invention
The present invention relates to a photoelectric converting apparatus and, more particularly, to a photoelectric converting apparatus having a plurality of photoelectric converting devices each of which can accumulate photocarriers onto a control electrode and can execute a photoelectric conversion and having at least a part of the photoelectric converting devices.
2. Related Background Art
As a photoelectric converting apparatus, there has been known an apparatus in which a light shielding layer is formed on a plurality of photoelectric converting devices formed on a semiconductor substrate so as to cover at least a part of the photoelectric converting elements and such a photoelectric converting device which has been covered by the light shielding layer is allowed to act as a dark pixel (for instance, U.S. Pat. No. 4,879,470).
A dark output compensation will now be briefly explained hereinbelow. FIG. 1A is a schematic constructional diagram of a conventional photoelectric converting apparatus having a dark output compensating function.
In the diagram, a sensor section 701 comprises: cells 7S.sub.1 to 7S.sub.n of an opening portion to execute a photoelectric conversion; and a cell 7S.sub.d of a light shielding portion to obtain a dark reference output.
Signals of the cells are sequentially generated by a scan section 702 and are supplied to a dark output compensation section 703. The dark output compensation section 703 subtracts the dark reference signal component of the cell 7S.sub.d from the signals of the cells 7S.sub.1 to 7S.sub.n and generates a resultant output signal.
Since an output of the sensor cell 7S.sub.d of the light shielding portion corresponds to a dark current of the sensor cell, by subtracting the dark reference signal component of the cell 7S.sub.d from the signals of the cells 7S.sub.1 to 7S.sub.n, a noise component by the dark current is eliminated. Consequently, a photoelectric conversion signal which accurately corresponds to incident light can be obtained.
As a dark output compensation section 703, a clamping circuit can be used or a sample and hold circuit to keep the dark reference signal of the cell 7S.sub.d and a differential circuit to calculate differences between the dark reference signal and the signals of the cells 7S.sub.1 to 7S.sub.n can be also used.
Explanation will now be made with reference to FIG. 1B which diagrammatically shows one pixel of photoelectric converting devices of the type which accumulates photocarriers onto a control electrode of a semiconductor transistor. Each of the photoelectric converting devices is constructed on a buried layer 26 formed on a semiconductor substrate 27 and comprises epitaxial layers 24, base layers 25, and device separating layers 23. A transparent insulating layer 22 is formed over each of the photoelectric converting devices. Further, the photoelectric converting device which functions as a dark pixel is covered by a metal light shielding layer 21 formed in or on the insulating layer 22.
There is a case where the photoelectric converting device having the light shielding layer which has been provided in correspondence thereto is hereinafter called a dark pixel and the photoelectric converting device to which a photosignal or optical information can enter is called a light pixel.
In the conventional photoelectric converting apparatus as shown in FIG. 1B, there are many cases where the light shielding layer 21 is also used as a power source line of V.sub.cc or the like. Therefore, a parasitic capacity is formed between the light shielding layer and the base region and a difference occurs between the base capacities of the dark pixel and the light pixel. Such a phenomenon will be described by using an equivalent circuit (FIG. 2) of a linear sensor in which BASIS (Base Store Image Sensor) are one-dimensionally arranged and a timing chart (FIG. 3) when such a linear sensor operates.
When a clock .phi..sub.CR rises at time t.sub.1, transistors M.sub.41 to M.sub.4n are simultaneously turned on and all of temporary accumulation capacitors C.sub.1 to C.sub.n are reset to VCR. When a clock .phi..sub.T rises at time t.sub.2, transistors M.sub.31 to M.sub.3n are turned on, transistors Q.sub.1 to Q.sub.n of a sensor section are turned on, and photo-signals accumulated in base capacitors Cb.sub.1 to Cb.sub.n are read out to the capacitors C.sub.1 to C.sub.n, respectively. After that, a clock .phi..sub.BR trails at time t.sub.3, transistors M.sub.11 to M.sub.1n are turned on, and the bases of the sensor section are reset (complete reset). Further, at time t.sub.4, a clock .phi..sub.ER rises, transistors M.sub.21 to M.sub.2n are turned on, emitters of the transistors Q.sub.1 to Q.sub.n are reset to VER, and the sensor is transiently reset.
After completion of the reading into the temporary accumulation capacitors and resetting of the sensor, the sensor starts the accumulating operation and the accumulated signals are read out of the temporary accumulation capacitors to an output terminal V.sub.out. That is, when a clock .phi..sub.HR rises at time t.sub.5 and a transistor M.sub.6 is turned on, an output line L is reset. When an output SR.sub.1 from a shift register SR rises at time t.sub.6, a transistor M.sub.51 is turned on and the signal is read out of the capacitor C.sub.1 to the output line. By repeating such a reading operation only the a number of times equal to the number of pixels, the reading operations are completed.
When considering the nth pixel in the reading operation from the sensor to the temporary accumulation capacitor, assuming that a long enough reading time has been given, an output voltage VE at the emitter terminal of the transistor Q.sub.n is expressed as follows: ##EQU1## where, A: pixel area
i.sub.p : photo current density PA0 t.sub.s : accumulation time PA0 Cb.sub.n : temporary accumulation capacity of the nth pixel
Therefore, as mentioned above, if temporary accumulation capacities Cb of the dark pixel and the light pixel differ, an output of the dark pixel differs from an output in, the dark state., of the light pixel, so that there is there occurs the case where a problem that the output of the dark pixel cannot be used as a black reference.
FIG. 4 shows a photoelectric converting apparatus in which pixels are two-dimensionally arranged. Reference numeral 101 denotes a vertical driving line; 102 a vertical scanning circuit; 103 a vertical signal line; 104 a switch means; 105 a horizontal signal line; 106 a horizontal scanning circuit; 107 buffer means; 108 horizontal signal line resetting means; and 110 a light shielding layer. Even in the photoelectric converting apparatus with the above structure, in the case where dark pixels are arranged like S.sub.11 to S.sub.1n, each base capacity of the dark pixels S.sub.11 to S.sub.1n is large and differs from a base capacity of the light pixel, so that the same problem as in the linear sensor occurs.